The present invention relates to a keyboard for use in a typewriter or similar office machine the keyboard being of the type provided with a plurality of keys each having an associated code card or mask provided with apertures defining light paths.
In a keyboard of the type contemplated by the invention, each code card is actuated by a drive mechanism in response to actuation of its associated key so that the card performs a switching movement which temporarily blocks or opens beam paths between light sources and light sensors, the switching movement occurring at a speed which is independent of the manner in which the key is actuated and taking place only one time through the beam paths in response to each key actuation.
Such a keyboard, which is used in conjunction with a teletype machine and one example of which is disclosed in the periodical "elektronik aktuell" [present-day electronics] No. 1/1976, is composed essentially of individual interchangeable components which can be inserted into a keyboard frame. Each individual component includes a plurality of individual parts. The most important operating parts of these individual key components include: an over-the-center leaf spring, a packet of two springs being required for each key component to provide optimum efficiency, which is acted upon substantially in the longitudinal direction by a key element, such as a key shaft, and which is capable of bistably assuming either one of two positions; and a mask which is acted upon by the leaf spring to effect a snap-action movement and which cooperates with a stationary coding plate. The coding plate is formed by a two-part receiving insert and one such plate is provided for each key, the mask being guided between the two parts of the insert. These receiving inserts for all of the coding plates are distributed over the entire width of the keyboard to form, together with perforations, the beam paths between the light sources and light sensors.
Such a known optically coded keyboard, which improves the operation of a prior art optical keyboard of the type disclosed in German Auslegeschrift No. 2,135,440, and British Pat. No. 1,340,305, with respect to noise and switching speed, achieves the purpose for which it was created, according to which the snap mask, which may constitute either a coding slide or a covering slide, is to intersect the beam paths only once during each key actuation and is to undergo a control movement which is independent of the speed and duration of the manual key actuation, the control movement always being effected at approximately the same speed. However, it still has certain drawbacks which will be explained in detail below.
In addition to the fact that the large number of individual parts required for such a keyboard cause its manufacturing and installation costs to exceed acceptable levels for economical electric office typewriters, it also has operational features which do not seem to make it optimally applicable for use in normal correspondence typewriters. The packet of over-the-center leaf springs, which in the production state is linear and in the installed state is clamped in a bowed form, is deformed by laterally arranged supporting tongues, when a key is manually actuated in the longitudinal direction, so that it takes on an S shape and the point of contact of the leaf spring packet is displaced along the supporting tongues. Then, after gradually passing over a dead point, the packet of leaf springs switches over to an arcuate form with a curve in the other direction and thus displaces the snap mask through the beam paths. The result is that the time of the snapping movement of the snap mask is not dependent solely upon the actuating stroke of the key, but is mainly dependent on the accuracy of the positioning of the supporting tongues and on the tensioning state of the leaf spring packet. A depth movement of the key element which is uniform for both directions of snap of the snap mask is therefore realizable only if the leaf spring packet is absolutely linear in the production state, this being necessary to produce identical tensioning forces in the packet for both stable positions in the installed state. However, such precise manufacture of a leaf spring packet is very difficult to attain on a mass production basis.
Moreover, sensitive users of such a known keyboard often consider it a drawback that the leaf spring packet, which for every actuation of a key performs only one snapping movement, can naturally emit only one response signal to be noted by touch or ear as a confirmation that the particular function has been actuated, i.e. when the key is being depressed. However, users who utilize the keys of a keyboard as resting points for their fingers when the keys are in the raised rest position, which helps to increase their typing speed, often require a similarly noticeable signal as an indication that the key has returned to the raised rest position and is therefore available for renewed function actuation.